An image sensor is a fundamental component of any device that measures or captures a spatial, frequency and/or intensity distribution of light to which it is exposed. An example of a system using such an image sensor is a digital camera system (irrespective of whether the system captures still or moving images).
The charge-coupled device (CCD) remains the most popular technology for implementing an image sensor. A competing technology is the CMOS image sensor.
It is known to provide structures on an image sensor to prevent pixels from blooming. A pixel is said to bloom when the level of electrons in the well rises above the point of being full (a.k.a. saturation) and electrons overflow/spread into adjacent pixels, which skews their accumulation of electrons.
In the case of a CCD image sensor, blooming prevention typically takes the form of providing an anti-blooming channel next to each vertical CCD (VCCD). If any of the pixels serviced by the VCCD blooms, then the overflowing electrons will spill into the anti-blooming channel and be drained away. It is further known to trigger reading of the entire CCD image sensor upon sensing the presence of electrons in any of the anti-blooming channels. This is known as electronic shuttering.
CMOS image sensors are inherently less prone to blooming because voltage is converted to charge at each pixel, and because it is not necessary to use shift registers (a VCCD and its horizontal counterpart, namely an HCCD) to bucket-brigade charge to the point of charge-to-voltage conversion. In addition, it is known to use the reset transistor present at each pixel as pixel-specific anti-blooming circuit.
Another difference between a CMOS and a CCD image sensor is that pixels on a CMOS sensor are individually addressable such that one or a few of the pixels can be read without having to read all of the pixels. This permits a CMOS image sensor to support windowing or window of interest (WOI) reading. Windowing is to be understood as reading a predetermined window (representing fewer than all of the pixels) from the image sensor without having to read all of the pixels. A user selects the window either by calculation or by reviewing a test image and then making the window selection. In contrast, CCD image sensors according to the Background Art have no provision for individually addressing one or more, but fewer than all, pixels, and so do not support windowing.